Keyboard is one of the most common input devices. It is widely used on various terminal units, such as computers, etc. An operator can input various instructions and data with a keyboard to a terminal unit, and can utilize the keyboard and a display conveniently communicate with the terminal unit. As different kinds of portable devices emerge, terminal units are limited in size, and accordingly the number of keys on the keyboard on such terminal units is limited. For example, on a portable device such as a Personal Digital Assistants (PDA) or a mobile phone, the number of keys on the keyboard (or keypad) is very small. Consequently, when running some application programs on such a device, the situation may arise, that required for using some specific keys, hut the key is not provided for on keyboard (or keypad). In addition, since the function of each key on the keyboard is fixed, many application programs are not compatible well with such a terminal unit.